There are two main types of Modbus protocols: Modbus TCP/IP, which is used for networks that run over the TCP/IP protocol, and Modbus RTU, which is used for serial communication. Both serve the same function, but are optimized for their respective communication medium.
Modbus TCP/IP vs Modbus RTU
While TCP/IP is more suitable for network applications and enables faster data transmission over longer distances, RTU is preferred for serial communication due to its simplicity and reliability.
Modbus RTU
Modbus RTU (Remote Terminal Unit) is one of the most common implementations of the Modbus protocol, which is designed for serial communication and is widely used in industrial environments for connecting electronic devices. Unlike Modbus TCP/IP, which is designed for compatibility with Ethernet and TCP/IP networks, Modbus RTU is specifically tailored for serial devices and connections such as RS-485 and RS-232.
Modbus RTU is a binary protocol that provides a simple and efficient way for devices, often called slaves, to communicate with a control device, the master. It is a synchronous protocol, as each 8-bit byte in a message is sent in sequence and framed by a start and stop bit. This simple structure increases the speed of data transmission in particular, making it very effective for real-time applications.
Modbus TCP/IP
Modbus TCP/IP is an extension of the traditional Modbus protocol that allows Modbus communication to take place over a TCP/IP network. This adaptation not only preserves the simplicity and reliability of the original (serial) Modbus RTU protocol, but also enables integration into modern network architectures, making it more suitable for large-scale and Internet-based applications.
Basically, Modbus TCP/IP uses the same model as the classic Modbus, namely a client-server (or master-slave) model. However, unlike Modbus RTU, which transmits data over serial connections, Modbus TCP/IP does so over TCP/IP networks. This means that it can use Ethernet connections and be forwarded via the Internet.
Modbus ASC II
Modbus ASCII (American Standard Code for Information Interchange) is another variant of the Modbus protocol for serial communication that encodes data as ASCII characters, as opposed to Modbus RTU, which encodes data in binary. Modbus ASCII is particularly useful in environments where data transparency and readability take precedence over data transmission speed.
Modbus ASCII represents each byte of data as two ASCII characters, which increases the size of the message packet but makes monitoring and troubleshooting easier as the contents of the packet can be read without the need for special software to decode it. This can be particularly useful for troubleshooting and in training environments, where understanding the data flow can help to diagnose problems more quickly.